[swift-evolution] [Proposal draff] abstract classes and methods

[Matthew Johnson]

> On Jan 7, 2016, at 12:42 PM, David James via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Not sure the language direction should encourage inheritance based structures, which abstract classes and methods do. That’s not to say that inheritance is dead, but that a modern language should encourage and support compositional patterns rather than inheritance based patterns. 

+1 to these comments.  This proposal notes that protocols would provide very similar behavior if they were allowed to have stored properties.  That has already been discussed a little bit.  I think it would be best to decide whether that will happen or not before introducing abstract classes and methods.

Matthew

> 
>> On Jan 7, 2016, at 7:19 PM, charles--- via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> +1
>> 
>> I have loads of "pretend abstract" classes littered with stuff like this:
>> 
>>    var boo:Bool! { return nil /*DUMMY*/ }
>> 
>> It takes a significant amount of energy atm to plan a Swift project because there are quirky differences between: protocol / subclass / class extension. It's not so straightforward to pick the most appropriate one.
>> 
>> 
>> 
>> Sent from my iPhone
>> 
>>> On Jan 7, 2016, at 9:55 AM, David Scrève via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> # Abstract classes and methods
>>> 
>>> * Author(s): David Scrève
>>> 
>>> ## Introduction
>>> 
>>> When developing framework and reusable, we need to develop classes that are partially 
>>> abstract with partial implementation. Protocol and protocol extensions provide this, but 
>>> they cannot have attributes as classes have.
>>> A partial class combines the behavior of a class with the requirement of implementing methods
>>> in inherited class like protocols.
>>> 
>>> Swift-evolution thread: [link to the discussion thread for that proposal](https://lists.swift.org/pipermail/swift-evolution <https://lists.swift.org/pipermail/swift-evolution>)
>>> 
>>> ## Motivation
>>> Like pure virtual methods in C++ and abtract classes in Java and C#, frameworks development 
>>> sometimes required abstract classes facility.
>>> An abstract class is like a regular class, but some methods/properties are not implemented 
>>> and must be implemented in one of inherited classes.
>>> An abstract class can inherit from other class, implements protocols and has members 
>>> attributes as opposite from protocols.
>>> Only some methods and properties might be abstract.
>>> The goal of abstract classes is to encapsulate a generic behavior that may need some 
>>> specific implementation methods which are not known in abstract class. This behavior 
>>> requires attributes that are used by internal abstract class method.
>>> 
>>> Example : 
>>> Considere a generic RESTClient that is included in a framework : 
>>> 
>>> ```swift
>>> class RESTClient {
>>> 
>>>   var timeout = 3000
>>> 
>>>   var url : String {
>>>       assert(false,"Must be overriden")
>>>       return ""
>>>   }
>>> 
>>>   func performNetworkCall() {
>>>       let restURL = self.url
>>>       print("Performing URL call to \(restURL) with timeout \(self.timeout)")
>>>   }
>>> }
>>> 
>>> ```
>>> 
>>> And an implementation : 
>>> ```swift
>>> class MyRestServiceClient : RESTClient {
>>>   override var url : String {
>>>       return "http://www.foo.com/client <http://www.foo.com/client>"
>>>   }
>>> 
>>> }
>>> ```
>>> 
>>> As you can see, url properties must be implemented by inherited class and should not be 
>>> implemented by ancestor.
>>> As workaround, we have added assertion, but this error is only detected at runtime and not 
>>> at compile time and might create crash for end-user.
>>> 
>>> ## Proposed solution
>>> We propose to add a new keyword to indicate that a method or a property is abstract and 
>>> not implemented in current class.
>>> This indicates that method or properties must be implemented in inherited class that can 
>>> be implemented.
>>> We propose the keyword abstract that must be added to class and property/method : 
>>> 
>>> ```swift
>>> abstract class RESTClient {    
>>>    var timeout = 3000
>>> 
>>>   abstract var url : String { get }
>>> 
>>>   func performNetworkCall() {
>>>       let restURL = self.url
>>>       print("Performing URL call to \(restURL) with timeout \(self.timeout)")
>>>   }
>>> }
>>> ```
>>> 
>>> And an implementation : 
>>> ```swift
>>> class MyRestServiceClient : RESTClient {
>>>   override var url : String {
>>>       return "http://www.foo.com/client <http://www.foo.com/client>"
>>>   }
>>> 
>>> }
>>> ```
>>> 
>>> ## Detailed design
>>> An abstract class cannot be instanciated. 
>>> 
>>> If a class contains one or more abstract methods/properties, it must be declared abstract.
>>> 
>>> A class that inherits from abstract must be declared abstract if it does not implements 
>>> all inherited methods/properties.
>>> 
>>> If you try to implement an abstract class or a inherited class that implements partially 
>>> abstract methods/properties, you will get a compiler error.
>>> 
>>> As for override keyword, abstract properties apply on setter, getter and observers. 
>>> 
>>> When declaring an abstract property, you must specify which methods must be implemented : 
>>> get, set, didSet, willSet. 
>>> 
>>> If you do not specify anything, only setter and getter are made 
>>> abstracts as below : 
>>> 
>>> ```swift
>>>   abstract var url : String
>>> ```
>>> 
>>> Observers provides default empty implementation.
>>> 
>>> Type is mandatory for abstract properties since it cannot be inferred.
>>> 
>>> ## Impact on existing code
>>> This change has no impact on existing code, but might change the ABI that is being 
>>> stabilizing in Swift 3.0.
>>> 
>>> ## Alternatives considered
>>> As first reading, it seems that protocols and protocol extensions might fit the need. It 
>>> actually does not because abstract classes can have attributs and properties that 
>>> protocols does not support.
>>> 
>>> An alternative solution would be to add attributes to protocols and protocol extensions, 
>>> but this might break compatibility with Objective-C runtime.
>>> 
>>> 
>>> 
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
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> 
> David James
> 
> 
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